Graphene is a two-dimensional (2D) sheet structure of sp2-bonded carbon atoms with unique electronic, chemical and mechanical properties. The significance of these unique properties are now just being realised, with graphene-based sheets being applied in various applications such as energy storage, catalysis, sensing, filtration and composites.
A considerable amount of research has been directed toward forming various graphene-based structures. One particular field of research showing great promise is the development of graphene-based film structures.
A number of techniques have been developed for producing graphene-based films. For example, graphene-based films have been prepared by filtration of graphene-based suspensions. Filtration has been widely used for manufacturing film-like structures such as writing paper since ancient times, but it has only recently been applied in the fabrication of graphene-film materials.
So called “graphene paper films” are typically formed on a membrane filter by vacuum filtration of partially reduced or reduced graphene dispersions. The resulting dry graphene paper is self-supporting with a degree of flexibility and can be peeled off from the membrane filter for subsequent use. Its structure is made up of closely packed laminated graphene-based sheets and can exhibit a conductivity of about 7,200 S/m at room temperature.
Despite such dry graphene-based paper films having good utility in certain applications, the relatively close packing of the graphene-based sheets results in a small inter-sheet separation distance (similar to that of graphite) having a packing density of about 1.49 g/cm3, which in turn presents a number of limitations including limited energy storage properties and a relatively low specific surface area.
Recently, WO 2012/006657 reported on new graphene-based gel film structures having graphene-based sheets arranged in a substantially planar manner relative to each other and being at least partially separated by a liquid medium such as water. The graphene-based gel films described are said to exhibit enhanced properties, relative to their dry paper counterparts, such as improved mechanical strength and electrical conductivity.
The gel films are also said to have a more open pore structure with a highly accessible surface area of individual graphene-based sheets.
Notably, the liquid medium retained in the gel films is believed to function as an inter-sheet spacing element thereby preventing the graphene-based sheets from becoming undesirably too closely packed.
Despite advances provided by these graphene-based gel films, the properties exhibited by them are by in large believed to be reliant upon their packing density, and this is inherently fixed depending upon the nature of the graphene-based material and liquid medium employed. In other words, the properties of a given graphene-based gel film are substantially fixed and predetermined by the graphene material and liquid medium used. Accordingly, to date there has been limited opportunity to adjust the properties of a given graphene-based gel film.
The liquid medium present within a graphene-based gel film can of course be removed, for example through evaporation. However, it has been found that the liquid medium tends to be removed unevenly across the film resulting in undesirable deformation of the film structure. For example, edge deformation of the film can occur due to preferential and non-uniform removal (relative to the remainder of the film) of the liquid medium from the film. Removing liquid medium from gel films to adjust the film properties has therefore to date not been particularly effective.
Accordingly, there remains an opportunity to develop a method for preparing graphene-based films that enables improved control over the resulting film structure and properties. By providing such control there also remains an opportunity for providing unique graphene-based films having improved properties.